What are the USB 3.0 Link Power Management states?
There are four Link power states in USB 3.0, which are explained below:
U0: Fully Powered; Active State for data transfer.
U1: Standby with Fast Recovery. No data transfer in this state and the power consumed is less than U0.
If there is no bus activity, Host/device can issue a request to go into U1 state. The other partner can accept or reject the request. If the other partner accepts, the link enters the U1 state. Otherwise, it remains in the U0 state. Later, if any link partner requests to access the link, it sends the U1 LFPS Exit (LFPS is Low Frequency Periodic Signaling) command to exit the U1 state. Finally, the link comes back to U0 from U1. The transition time from U1 to U0 is within microseconds.
U2: Standby with Slow Recovery. The power saving on the link in U2 is more than U1.
The link partner can enter U2 in two ways as follows:
a) Enter to U2 from U0: If there is no bus activity, the Host/device can issue a request to go into the U2 state. The other partner can accept or reject the request. If the other partner accepts the request, the link goes to U2 state; otherwise, it remains in the U0 state.
b) Enter to U2 from U1:
A port in the U1 state enters U2 directly if the following two conditions are met:
1. The port’s U2 inactivity timer is enabled.
2. The U2 inactivity timer times out and no U1 LFPS exit signal is received.
The exit process is the same as U1 but the LFPS Exit duration for exiting the U2 state is larger than U1. The transition time from U2 to U0 is within microseconds to milliseconds.
U3: Suspend State. This is the highest link power saving state. Only the Host requests the device to enter the U3 state. The device must accept the request and enter the U3 state. Either the Host or the device can exit the U3 state to U0 state. The transition time from U3 to U0 is in milliseconds.